This program project brings together investigators from a variety of backgrounds attracted to the keratinocyte as a model for the study of differentiation. Both in vivo and in vitro models of keratinocyte differentiation will be utilized. There are five projects in this proposal. Project 1 will examine the role of phospholipase C-gamma-1 (PLC-gamma1) in calcium induced keratinocyte differentiation, focusing on the mechanisms by which calcium activates PLC-gamma1, the requirement for such activation in the ability of PLC-gamma-1 to interact with other components of the calcium signaling pathways, and the role of PLC-gamma-1 in vivo in calcium regulated barrier function. Project 2 will examine the role of CD44 and its ligand hyaluronic acid in regulating calcium signaling and differentiation focusing on three pathways: Rho activated Rhokinase and Rac1 activation of PLC-gamma-1 each contributing to the regulation of IP3 receptor mediated intracellular calcium mobilization, and src kinase regulation of the cortactin mediated cytoskeletal reorganization required or Keratinocyte differentiation. Project 3 will evaluate the role of the Golgi calcium pump in normal calcium signaling, and examine the pathophysiologic basis for Hailey-Hailey Disease, a disease caused by mutations in the Golgi calcium pump. Project 4 will examine the coordinate regulation of sphingolipid production, in particular ceramide and glucosylceramide, by the same liposensing nuclear hormone receptors being studied in project 5. Project 5 will examine the molecular mechanisms by which liposensing nuclear hormone receptors regulate differentiation, focusing on their ability to regulate AP-1 transcription factors. The Cell Culture/Tissue Preparation Core will continue to provide the cells, organ cultures, and epidermal preparations required by all projects. The Microscopy and Molecular Histology Core will continue to provide in situ hybridization and immunochemical localization of proteins of interest, the light and electron microscopic analyses critical for the in vivo studies being pursued by most of the investigators, and assist with the confocal microscopic studies proposed by most investigators using the equipment in the SFVAMC Imaging Core. The Molecular Resources Core is new to our Program Project, and will assist our investigators with the new equipment being installed at the SFVAMC for real time quantitative PCR, DNA microarray studies, and mass spectrometry for protein/peptide identification. The members of the Program Project will continue to meet on a regular basis, sharing ideas, reagents, and expertise enriching the collaboration that has developed over the years of working together. This proposal, then, represents a continuation of our ongoing successful effort at building a cohesive, interdisciplinary group aimed at unraveling the regulation of keratinocyte/epidermal differentiation.